Polyelectrolyte brushes consist of charged polymers that are tethered to a surface by one chain end. Swelling behavior of polyelectrolyte brushes under aqueous conditions has attracted researchers’ attention from viewpoints of fundamental and applied sciences since they show a rich behavior with many potential applications for tuning surface properties. With this PhD project, I aim to shed light on the role of three underinvestigated aspects of the swelling behavior of polyelectrolyte brushes: the effect of brush density profile, the role of nonelectrostatic interactions, and the specific ion effects. This project is based on three publications corresponding to these three subjects.
In the first study, vertical non-uniformity is studied experimentally with a major focus on the gradient of dissociation degree in a weak polyelectrolyte brush. Here, by employing a combination of ellipsometry and quartz crystal microbalance with dissipation monitoring (QCM-D), it is demonstrated that dissociation degree of a weak polyelectrolyte brush increases with increasing the distance from the substrate and this gradient of dissociation degree becomes more pronounced with decreasing ionic strength. In the second study, the role of nonelectrostatic, polymer-polymer and polymer-solvent, interactions are investigated experimentally and theoretically. By designing polyelectrolyte brushes with different side groups but similar chain length, dispersity, grafting density, and charge fraction, nonelectrostatic effects were systematically studied. Fundamentally different swelling behaviors are observed experimentally depending on the nonelectrostatic interactions, and these effects are approved theoretically by the results of a mean-field model. The third study focuses on the impact of ion type. The evidence for the existence of two different mechanisms for ion specificity are demonstrated experimentally by studying the swelling behavior of a strong polycationic brush with various counterions. Moreover, by employing the theoretical model in combination with the experimental results, the contribution of these mechanisms are analysed with respect to brush and medium properties. As an extension to the third study, two sets of preliminary results, mainly about the ion- polyelectrolyte interactions, are also discussed which raise new questions and suggest ideas for future works.